Spark-ignition (petrol) engines — contents drawn during the suction (intake) stroke During the suction stroke of a conventional petrol engine with a carburettor or port fuel injection, what enters the cylinder?

Introduction / Context:
This question probes your understanding of the intake process in spark-ignition (SI) petrol engines. Knowing what actually enters the cylinder during the suction stroke is foundational to combustion, emissions, and performance concepts.

Given Data / Assumptions:

• Petrol (gasoline) engine operating on the Otto principle.
• Mixture preparation by a carburettor or by port fuel injection upstream of the intake valve.
• Throttle controls mass flow and manifold pressure.

Concept / Approach:
In a typical SI engine, fuel is vaporised and mixed with air before entering the cylinder. In carburetted systems, the venturi draws fuel into the airstream. In port fuel injection, the injector sprays fuel into the intake port where it mixes with incoming air; the intake valve then admits this premixed charge. This differs from compression-ignition Diesels, which ingest air only on the intake stroke and inject fuel directly into hot compressed air near the end of compression.

Step-by-Step Solution:
Identify the engine type: petrol SI with upstream mixture formation.During the suction stroke, piston descends and intake valve opens.Manifold depression draws in the prepared air–fuel mixture.Therefore, the correct entry to the cylinder is air and fuel together.

Verification / Alternative check:
Direct-injection petrol engines (GDI) may inject fuel into the cylinder; however, many still admit air (sometimes air only in specific stratified modes) and then add fuel during the intake or compression event. The mainstream historical and educational baseline for “petrol engine” intake is air–fuel mixture via carburettor or PFI.

Why Other Options Are Wrong:

• Only fuel: impossible; combustion requires oxygen and the engine cannot ingest fuel alone.
• Only air: that is the Diesel intake case, not the classic petrol carburetted/port-injected case.
• None of these: incorrect because a clear correct choice is available.

Common Pitfalls:
Confusing SI with CI operation, or assuming all modern engines are GDI and thus always draw “air only.” Even with GDI, the intake stroke contains air and often some fuel addition depending on the strategy.

Final Answer:
air and fuel